Radiation-induced defects in quartz. III. Single-crystal EPR,ENDOR and ESEEM study of a peroxy radical

The X- and W-band single-crystal electron paramagnetic resonance spectra of an electron-irradiated natural quartz permit quantitative analysis of a ²⁹Si hyperfine structure (A ~12.6 MHz) and an ²⁷Al hyperfine structure (A ≤ 0.8 MHz) for a previously reported hole-like center. The ²⁹Si hyperfine structure arises from interaction with two equivalent Si atoms and is characterized by the direction of the unique A axis close to a Si–O bond direction. The ²⁷Al hyperfine structure, confirmed by pulsed electron nuclear double resonance and electron spin echo envelope modulation spectra, is characterized by the unique A axis approximately along a twofold symmetry axis. These ²⁹Si and ²⁷Al hyperfine data, together with published theoretical results on peroxy radicals in SiO₂ as well as our own density functional theory (DFT) calculations on model peroxy centers, suggest this hole-like center to have the unpaired spin on a pair of oxygen atoms linked to two symmetrically equivalent Si atoms and a substitutional Al³⁺ ion across the c-axis channel, a first peroxy radical in quartz. The nuclear quadrupole matrix P also suggests that the Al³⁺ ion corresponds closely to the diamagnetic precursor to the [AlO₄]⁰ center. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Millennial timescale resolution of rhyolite magma recharge at Tarawera volcano: insights from quartz chemistry and melt inclusions

Most rhyolite eruption episodes of Tarawera volcano have emitted several physiochemically distinct magma batches (∼1–10 km³). These episodes were separated on a millennial timescale. The magma batches were relatively homogeneous in temperature and composition at pumice scale (>4 cm), but experienced isolated crystallisation histories. At the sub-cm scale, matrix glasses have trace element compositions (Sr, Ba, Rb) that vary by factors up to 2.5, indicating incomplete mixing of separate melts. Some quartz-hosted melt inclusions are depleted in compatible trace elements (Sr, Ti, Ba) compared to enclosing matrix glasses. This could reflect re-melting of felsic crystals deeper in the crystal pile. Individual quartz crystals display a variety of cathodoluminescence brightness and Ti zoning patterns including rapid changes in melt chemistry and/or temperature (∼50–100°C), and point to multi-cycle crystallisation histories. The Tarawera magma system consisted of a crystal-rich mass containing waxing and waning melt pockets that were periodically recharged by silicic melts driven by basaltic intrusion. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Deformation of experimentally shock-loaded quartz powders: X-ray line broadening studies

Quartz powders (mean grain size: 22 μm) were pressed into sample discs of different green densities (ρ₀=1.65; 1.85 and 2.05 g/cm³) and subjected to shock pressure between 1.5 and 17.0 GPa. Peak shock pressures were determined by the impedance method using the Hugoniot curves of steel and quartz powders. Fourier techniques were used to analyse the line broadening of 5 X-ray reflections in each case. The Fourier coefficients were subjected to a Rothman-Cohen correction before further processing. The coherent domain size decreases abruptly from several thousand Ångström in the unshocked state to about 800 Å at 1.5 GPa, and reaches a constant mean value of about 200 to 300 Å at dynamic pressures of about 4 GPa. At very high dynamic pressures (?15 GPa) there is further fragmentation to very small domain size. There is no systematic correlation between sample density and coherent domain size produced by the shock event. A more or less linear dependency exists between microstrain and pressure for each starting density of the quartz powder. The observation that microstrain decreases with increasing starting density could be explained by the fact that increasingly larger portions of the input energy are consumed to create fresh surfaces by comminution. Consequently, less energy will be available for strain strengthening. The stored energies in the shocked quartz powders are of the same order as the surface energy. Shock-treated quartz should therefore be suitable for accelerating any activated process such as sintering.     

The investigation of defects in shock-metamorphosed quartz

The shock-metamorphosed quartz exhibits thermal luminescence (TL) with maxima at 365 nm, 470 nm and 610–680 nm. By electron paramagnetic resonance (EPR) analysis E₁ type electron centers and hole centers have been found which originate from vacancies including those from the substitution of Al³⁺ and/or Fe³⁺, for Si⁴⁺. The EPR and TL spectra may be interpreted mainly in terms of vacancy type defects associated with dislocations in the crystal structure of quartz.     

High-pressure crystal chemistry of chrysoberyl,Al2BeO4: Insights on the origin of olivine elastic anisotropy

High-pressure crystal structure refinements and axial compressibilities have been determined by x-ray methods for the olivine isomorph chrysoberyl, Al₂BeO₄. Unlike silicate olivines, which are more than twice as compressible along b than along a, chrysoberyl (space group Pbnm) has nearly isotropic compressibility with β _a =1.12±0.04, β _b =1.46±0.05, and β _c =1.31±0.03 (all×10^?4 kbar^?1). The resultant bulk modulus is 2.42±0.05 Mbar, with K′ assumed to be 4. The axial compression ratios of chrysoberyl are 1.00:1.30:1.17, compared to axial compression ratios 1.00:2.02:1.60 for forsterite. These differences in compression anisotropy arise from differences in relative bond compressibilities. In chrysoberyl the average aluminum-oxygen and beryllium-oxygen bond compressibilities are similar, yielding nearly isotropic compression, but in silicate olivines octahedral cation-oxygen bonds are significantly more compressible than Si-O bonds, so that compression parallel to a is much more restricted than that parallel to b. The inherent anisotropy of the olivine structure is not, by itself, sufficient to cause anisotropic compression. It appears that in the case of olivine the distribution of cations of different valences, in conjunction with the structure type, leads to anisotropies in physical properties.     

The crystal chemistry of mordenites

Mordenite is a zeolite whose approximate composition is (Na₂, K₂,Ca)₄[Al₈Si₄₀O₉₆] 28 H₂O. Unit cell dimensions, determined by X-ray powder diffractometry for 35 natural samples, fell within the following ranges: a=18.052–18.168, b=20.404–20.527, c=7.501–7.537 Å. The indexed powder pattern of a typical sample is reported. Complete wet chemical analyses of 12 samples, partial analyses of three others, and 6 analyses from the literature reveal that mordenites vary only slightly in chemical composition. Si occupies 80 to 85% of the tetrahedra, and the exchangeable cations are mainly Na and Ca, with minor K. The lattice constant b is negatively correlated to the ratio Si/(Si+Al+Fe?).     

Soil chemistry in lithologically diverse datasets: The quartz dilution effect

National- and continental-scale soil geochemical datasets are likely to move our understanding of broad soil geochemistry patterns forward significantly. Patterns of chemistry and mineralogy delineated from these datasets are strongly influenced by the composition of the soil parent material, which itself is largely a function of lithology and particle size sorting. Such controls present a challenge by obscuring subtler patterns arising from subsequent pedogenic processes. Here the effect of quartz concentration is examined in moist-climate soils from a pilot dataset of the North American Soil Geochemical Landscapes Project. Due to variable and high quartz contents (6.2–81.7 wt.%), and its residual and inert nature in soil, quartz is demonstrated to influence broad patterns in soil chemistry. A dilution effect is observed whereby concentrations of various elements are significantly and strongly negatively correlated with quartz. Quartz content drives artificial positive correlations between concentrations of some elements and obscures negative correlations between others. Unadjusted soil data show the highly mobile base cations Ca, Mg, and Na to be often strongly positively correlated with intermediately mobile Al or Fe, and generally uncorrelated with the relatively immobile high-field-strength elements (HFS) Ti and Nb. Both patterns are contrary to broad expectations for soils being weathered and leached. After transforming bulk soil chemistry to a quartz-free basis, the base cations are generally uncorrelated with Al and Fe, and negative correlations generally emerge with the HFS elements. Quartz-free element data may be a useful tool for elucidating patterns of weathering or parent-material chemistry in large soil datasets.     

Cyrilovite from Italy: Structure and crystal chemistry

Summary The crystal structure of the cyrilovite from Bosa, Sardinia, Italy, a new locality for this rare mineral, has been refined in the P 4₁2₁2 space group to aR value of 0.053 for 907 observed reflections. The structure consists of sheets of Fe³⁺-centered octahedra sharing vertices to form a square net, in the center of which the 8-fold coordinated Na+ ion is located. The Na-Fe-O sheets are linked together by P0₄ distorted tetrahedra and by hydrogen bonds. The unit-cell parameters (a = 7.313 (2) Å,c = 19.315 (3) Å, V = 1033.0 (4) Å3,c/a 2.641) are substantially greater than those of wardite, the other endmember of the solid solution series. The chemistry is close to the end-member, but shows a limited solid solution of Al for Fe, and of Ca for Na; the latter may imply a yet poorly known possibility of vacancies in the 8-fold coordinated sites. The IR spectra show the presence of AI substituting for Fe³⁺ in the form of additional weak bands.[

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Cyrilovit aus Italien:Struktur und kristallchemie

Zusammenfassung Die Kristallstruktur von Cyrilovit aus Bosa, Sardinien, Italien - einem neuen Fundort dieses seltenen Minerals - wurde in Raumgruppe P 4₁2₁2 auf einen R-Wert von 0,053 für 907 beobachtete Reflexe verfeinert. Die Struktur besteht aus Schichten; Oktaeder mit Fe³⁺ als Zentralatom werden über Ecken zu einem quadratischen Netz verknüpft, wobei in der Mitte ein 8-koordiniertes Na+-Ion liegt. Die Na-Fe-O-Schichten werden durch verzerrte PO₄-Tetraeder und durch Wasserstoffbrückenbindungen verknüpft. Die Gitterparametera = 7,313 (2) Å,c = 19,315 (3) Å,V = 1033,0 (4) Å3,c/a = 2,641) sind wesentlich größer als jene des Wardits, dem zweiten Endglied dieser Mischkristallreihe. Die chemische Zusammensetzung kommt dem Endglied nahe, zeigt jedoch einen geringfügigen Ersatz von AI für Fe bzw. von Ca für Na; letzterer dürfte auf die bisher nur wenig beachtete Möglichkeit von Leerstellen in der 8-koordinierten Position hindeuten. Das IR-Spektrum zeigt aufgrund zusätzlicher schwacher Banden das Vorliegen des Ersatzes von Al für Fe³⁺.

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With 3 Figures     

High-temperature vesuvianite: crystal chemistry and surface considerations

A multi-methodical approach has been applied for characterizing the bulk and surface crystal chemical features of a high-temperature vesuvianite crystal from skarns of Mount Somma-Vesuvius Volcano (Naples, Italy). Vesuvianite belongs to the space group P4/nnc with unit cell parameters a = 15.633(1) Å, c = 11.834(1) Å and chemical formula (Ca_18.858 Na_0.028 Ba_0.004 K_0.006 Sr_0.005 □_0.098)_19.000 (Al_8.813 Ti_0.037 Mg_2.954 Mn_0.008 Fe_0.114²⁺ Fe_1.375³⁺ Cr_0.008 B_0.202)_13.511 Si_18.000(O_0.261 F_0.940 OH_7.799)_9.000. Structure refinement, which converges at R = 0.0328, demonstrates a strong positional disorder down the fourfold axes, indicating that the Y1 site is split into two positions (Y1A and Y1B) alternatively occupied. However, because of X4 proximity to Y1B and Y1A, X4 cannot be occupied if Y1B or Y1A are. Overall Y1 occupancy (Y1A + Y1B) reaches approximately 0.5, as common in vesuvianite and occupancy of Y1B site is extremely limited. Moreover, T1 position, limitedly occupied, accommodates the excess of cations generally related to Y position. A small quantity (0.202 apfu) of boron is sited at the T2 site that, like T1, is poorly occupied. The determination of the amount of each element on the (100) vesuvianite surface, obtained through X-ray photoelectron spectroscopy high-resolution spectra in the region of the Si_2p, Al_2p, Mg_1s, and Ca_2p core levels, evidences that a greater amount of aluminum and a smaller amount of calcium characterize the surface with respect to the bulk. Although both of these features require further investigation, we may consider the Al increase can be related to preferential orientation of Al-rich sites on the (100) plane. Furthermore, the surface structure of vesuvianite suggests that Al, Ca, and Mg cations maintain coordination features at the surface similar to the bulk. Silica, however, while presenting fourfold coordination, shows also a [1]-fold small coordinated component at binding energy 99.85 eV, due to broken Si–O bonds at the surface. The presence of eight- and nine-coordinated Ca cations is suggested by a large XPS feature resulting from the merging of Ca_2p3/2 and Ca_2p1/2 peaks at 348.45 and 352.05 eV, respectively.     

Springtime changes in snow chemistry lead to new insights into mercury methylation in the Arctic

Seasonal snow is an active media and an important climate factor that governs nutrient transfer in Arctic ecosystems. Since the snow stores and transforms nutrients and contaminants, it is of crucial importance to gain a better understanding of the dynamics of contaminant cycling within the snowpack and its subsequent release to catchments via meltwater. Over the course of a two-month field study in the spring of 2008, we collected snow and meltwater samples from a seasonal snowpack in Ny-Ålesund, Norway (78°56′N, 11°52′E), which were analyzed for major inorganic ions and some organic acids, as well as total, dissolved, bioavailable mercury (THg, DHg, BioHg, respectively) and monomethylmercury (MMHg) species. We observe a seasonal gradient for ion concentrations, with surface samples becoming less concentrated as the season progressed. A significant negative correlation between BioHg and MMHg was observed in the snowpack. MMHg was positively and significantly correlated to methanesulfonate concentrations. Based on these results, we propose a new model for aerobic methylation of mercury involving species in the dimethylsulfoniopropionate cycle.     

Radiation-induced defects in quartz. II. Single-crystal W-band EPR study of a natural citrine quartz

Single-crystal electron paramagnetic resonance (EPR) spectra of a natural citrine quartz without any artificial irradiation, measured at W-band frequencies (∼94 GHz) and temperatures of 77, 110 and 298 K, allow better characterization of three previously-reported Centers (#6, #7 and B) and discovery of three new defects (B′, C′ and G′). The W-band EPR spectra reveal that Centers #6 and #7 do not reside on twofold symmetry axes, contrary to results from a previous X-band EPR study. The W-band spectra also show that the previously reported Center B is a mixture of two defects (B and B′) with similar g matrices but different-sized ²⁷Al hyperfine structures. Center C′ has similar principal g values to the previously reported Center C but is distinct from the latter by a larger ²⁷Al hyperfine structure with splittings from 0.10 to 0.22 mT. Also, Center G′ has a similar g matrix to the previously reported Center G but a different ²⁷Al hyperfine structure with splittings from 0.41 to 0.53 mT. These spin-Hamiltonian parameters, together with observed thermal properties and microwave-power dependence, suggest that Centers #6 and #7 probably represent O₂³⁻ type defects. Centers B and B′ are probably superoxide radicals (O₂⁻) with the unpaired spin localized on the same pair of oxygen atoms around a missing Si atom but linked to a substitutional Al³⁺ ion each at different neighboring tetrahedral sites. Similarly, Centers G and G′ are most likely superoxide radicals with the unpaired spin localized on another pair of oxygen atoms around a missing Si atom and linked to a substitutional Al³⁺ ion each at different neighboring tetrahedral sites. Center C′ is probably an ozonide radical associated with a missing Si atom and linked to a substitutional Al³⁺ ion at the neighboring tetrahedral site. This study exemplifies the value of  high-frequency EPR for discrimination of  similar defect centers and determination of  small local structural distortions that are often difficult to resolve in conventional  X- and Q-band EPR studies.     

Dissolution of nanocrystalline fluorite powders: An investigation by XRD and solution chemistry

The effect of lattice disorder and mineral surface area on the reactivity of finely ground fluorite was studied on ball-milled powders. Structural information was provided by X-ray whole powder pattern modeling (WPPM). The mean size of coherent scattering domains decreases with milling time from 70 nm to ∼20 nm, whereas the density of lattice defects increases with both time and intensity of milling treatment, from 4 × 10¹⁵ m^-2 to 24 × 10¹⁵ m^-2. High resolution transmission electron microscopy (HRTEM) of ground fluorite grains shows several line defects and a general tendency of nanometric crystalline domains to agglomerate in larger grains.Solution chemistry was investigated using batch reactors with free drift of solution saturation state with respect to fluorite. Total surface area was measured by the Brunaver, Emmet and Teller (BET) method, and dissolution rates were measured at pH = 2 (HCl) and T = 295 K. In far from equilibrium conditions, dissolution rates normalized by BET area do not increase with the dislocation density. In near-equilibrium condition, however, measured stationary ionic product clearly increases with both time and intensity of milling treatment. Thermodynamic predictions of the solubility constant indicate negligible or little effect of total surface area. Consequently, the observed increase in the stationary ionic product can be related to the increasing lattice defect content. This confirms the significant role of dislocation outcrops on mineral dissolution in close to equilibrium conditions.     

Evolution of fluid chemistry in quartz compaction systems: Experimental investigations and numerical modeling

The evolution of fluid chemistry in compacting rock is controlled by coupled chemical processes and rock deformation. In order to characterize this evolution, we conducted water-rock interaction experiments using quartz aggregates at 150 °C and effective pressure of 34.5 MPa. A coupled fluid flow, chemical reaction, and creep compaction model is developed, in which both free-surface reaction and grain-contact dissolution are considered as system volume and porosity evolve.The direct experimental measurement and numerical modeling indicate that effective pressure has significant effects on pore-fluid chemistry. At the early stages of compaction, pore fluids are supersaturated with respect to bulk quartz. With increasing compaction and time, solute concentrations gradually decrease to saturated conditions. Supersaturation is caused mainly by dissolution of ultrafines and high-energy, unstable surfaces which are produced by stress concentrations at grain contacts during the very early stages of compaction. Grain-contact dissolution also contributes to the solute increase in pore fluid in the early stage of compaction, but the effect is small compared to that of ultrafines and unstable surfaces and only slight supersaturation can be produced by it. The gradual decrease in pore-fluid concentration is related to the mechanical removal of ultrafines by pore-fluid flow and the dissolution of ultrafines and unstable surfaces. It also results from the lessening of grain-contact dissolution.Pore fluids in compacting sedimentary basins of quartz sandstone are nearly saturated throughout most of diagenetic processes. Ultrafines and unstable surfaces produced by stress appear not to be the major sources of quartz cement.     

The crystal chemistry of roméite

Roméite (Ca, Fe, Mn, Na)₂(Sb⁵⁺, Ti⁴⁺)₂(O, OH, F)₇ is a rare mineral found in metamorphic iron-manganese deposits and in hydrothermal Sb-bearing veins. It is isostructural with the pyrochlore-group minerals of the general formula A_2–mB₂X_6–wY_1–n · pH₂O. The pyrochlore-group minerals are important Nb and Ta ores, and are also used as an actinide host phase in␣radioactive waste. The crystal chemistry of roméite from the type locality Praborna (Italy), from Massiac (France), and from four newly discovered localities in␣the Swiss Alps, and of “lewisite”, a questionable species related to roméite from Tripuhy (Brazil), is compared to that of pyrochlore. A wide range of substitutions has been observed including (1) independent substitutions on the A- and B-sites, and (2) coupled substitutions between the A- and B- and between the A- and Y- sites. Only the roméite from Massiac, derived from weathering of stibnite, contains significant H₂O (up to 14 wt %). The A-site vacancies in roméite appear to be controlled by the primary conditions of crystallization, and not by post-crystallization alteration. The Y-site chemistry of roméite varies from locality to locality; it can be dominated by F, OH, or be fully vacant. The “lewisite” octahedral crystals studied are a sub-microscopic mixture of roméite with a mineral structurally related to pyrochlore, which grows at the expense of roméite. Received: 5 March 1996 / Accepted: 18 October 1996     

Trace element crystal chemistry of mantle eclogites

Rare earth element (REE) concentrations have been measured using instrumental neutron activation analysis on clean separates of primary minerals from 11 eclogite samples from the Bobbejaan and Roberts Victor kimberlites, South Africa. Samples were selected to reflect minimal secondary alteration and represent a broad range of eclogite compositions from coesiteand corundum-grospydites through magnesian bimineralic eclogites. Correlations between REE concentrations and major-element compositions suggest that garnet and clinopyroxene crystal chemistry are the dominant control on REE distribution and that these approach solidstate equilibrium distributions. Reconstructed wholerock REE concentration variation with whole-rock major-element compositions are consistent with an origin by high-pressure igneous fractionation followed by reequilibration to lower temperatures at pressures in excess of three GPa.     

High-pressure crystal chemistry of MgSiO3 perovskite

A high-pressure single-crystal x-ray diffraction study of perovskite-type MgSiO₃ has been completed to 12.6 GPa. The compressibility of MgSiO₃ perovskite is anisotropic with b approximately 23% less compressible than a or c which have similar compressibilities. The observed unit cell compression gives a bulk modulus of 254 GPa using a Birch-Murnaghan equation of state with K set equal to 4 and V/V ₀ at room pressure equal to one. Between room pressure and 5 GPa, the primary response of the structure to pressure is compression of the Mg-O and Si-O bonds. Above 5 GPa, the SiO₆ octahedra tilt, particularly in the [bc]-plane. The distortion of the MgO₁₂ site increases under compression. The variation of the O(2)-O(2)-O(2) angles and bondlength distortion of the MgO₁₂ site with pressure in MgSiO₃ perovskite follow trends observed in GdFeO₃type perovskites with increasing distortion. Such trends might be useful for predicting distortions in GdFeO₃-type perovskites as a function of pressure.     

Contribution to the crystal chemistry of beryl

Chemical analyses and crystallographic and some optical data have been obtained for 28 samples of beryl from Bahia State, Brazil. The larger range of variability in the chemical composition is shown by Mg, Fe and Li. Sodium is the more diffuse alkali element. Potassium is always very limited. Calcium appears in noticeable amounts only in three samples. The Ti, Cr, Rb, and Cs elements were also tested. The samples studied here can be defined as sodium-potassium beryls with low alkali content. Unit cell parameters show the following ranges: a=9.210–9.245 and c=9.190–9.220 Å. From a statistical analysis of these data it may be seen that: an increase of Fe and (Fe+Mn+Mg) percentage has a positive correlation with a, but no influence on c, which in turn has a close positive correlation with Li and is negatively correlated with Be. Less negative correlations also exist between the pairs {Be, Li}, {a,Al}, {Al, Fe} and {Al, (Fe+Mn+Mg)}. A positive correlation also exists between sodium and the parameter a.